Hysteresis during transition into improved mode on biasing experiment under the electrode current control in Tohoku University Heliac

Electrode bias experiments were carried out in Tohoku University Heliac (TU-Heliac). This paper presents a comparison of the plasma behaviour in discharge of the electrode current ramped up with that ramped down to investigate the bifurcation and hysteresis phenomena. Bifurcation of the electrode current against the bias voltage, i.e. negative resistance and hysteresis between (1) electron stored energy and input power from the electrode, (2) fluctuation level of ion saturation current and radial electric field and (3) poloidal momentum damping force and poloidal Mach number were observed. The driving force in the region where the hysteresis was observed corresponded to those in the negative resistance phase and those were located at local maximum of the ion viscosity. Hysteresis between the poloidal Mach number and the poloidal damping force was observed. However, the difference in ion viscosity, which was obtained by subtracting the friction term from the total poloidal damping force, between the electrode current ramped up/down in the transition region was rather small. The data in the negative resistance region corresponded to those in the regime where the viscosity decreased against the poloidal Mach number.